Wi-Fi Radar via Over-the-Air Referencing: Bridging Wi-Fi Sensing and Bistatic Radar

TL;DR

This work addresses the challenge of achieving phase-coherent radar-like analysis in unsynchronized Wi‑Fi systems by introducing LoSRef, an OTA LoS-path referencing scheme that uses the Tx–Rx line-of-sight path as a stable delay and phase reference. By carefully preprocessing wideband CFR data from unmodified 802.11ax devices, forming a robust CIR, and aligning it to a common LoS reference, the method enables phase-coherent time-variant impulse response estimation and a delay–Doppler representation without wired references or dedicated antennas. The authors validate the approach experimentally with COTS hardware, demonstrating gait-range estimation and respiration-induced sub-wavelength motion, and show that phase-coherent processing preserves Doppler signs and enables more physically interpretable sensing than magnitude-based methods. The technique bridges Wi‑Fi sensing and bistatic radar, offering drop-in deployment and a pathway to radar-like ISAC capabilities using existing Wi‑Fi infrastructure.

Abstract

Wi-Fi sensing has attracted significant attention for human sensing and related applications. However, unsynchronized transmitters and receivers fundamentally preclude phase-coherent radar-like delay--Doppler analysis. By exploiting the line-of-sight (LoS) path, i.e., the earliest-arriving direct path, as an over-the-air (OTA) reference for delay and phase, we propose an OTA LoS-path referencing scheme, termed LoSRef, that enables delay calibration and phase alignment in unsynchronized Wi-Fi systems. Unlike conventional Wi-Fi bistatic radar systems that rely on wired reference signals or dedicated reference antennas, the proposed LoSRef-based framework bridges the long-standing gap between conventional Wi-Fi sensing and Wi-Fi radar, enabling phase-coherent bistatic radar-like operation in a drop-in Wi-Fi sensing configuration. Through human gait and respiration experiments in indoor environments, we demonstrate that phase-coherent channel impulse responses and corresponding delay--Doppler responses are obtained using only commodity Wi-Fi devices. This enables physically interpretable human motion sensing, including gait-induced range variation and respiration-induced sub-wavelength displacement, as well as the extraction of target-induced dynamics up to 20 dB weaker than dominant static multipath components.

Figures (19)

• Figure 1: Comparison of reference acquisition strategies. While (a) and (b) rely on dedicated reference hardware, the proposed OTA LoS-path referencing scheme (LoSRef) in (c) exploits the Tx--Rx LoS path as a delay reference, eliminating the need for auxiliary reference hardware. Moreover, (c) is compatible with a single-port Rx, whereas (a) and (b) require multi-port Rxs.
• Figure 2: Processing flow from measured CFR to the delay--Doppler response $s(\tau,\nu;t)$. The LoSRef exploits the Tx--Rx line-of-sight path as a physical delay and phase reference, enabling phase-coherent alignment of CIR across packet acquisitions.
• Figure 3: LoSRef: OTA LoS-path referencing for delay calibration and phase alignment. The dominant LoS-path component is used as an OTA reference to compensate unknown offsets caused by unsynchronized packet transmissions.
• Figure 4: A COTS Wi-Fi device used as the Tx or Rx.
• Figure 5: Impact of phase and power preprocessing on the CFR.